The present invention relates to low viscosity polymer polyols and to a process for their preparation. This invention also relates to a process for the production of polyurethane foams from these low viscosity polymer polyols, and to the resultant foams
Polymer polyol compositions suitable for use in producing polyurethane foams, elastomers and the like, and the polyurethanes, are commercial products. The two major types of these polyurethane foams are termed slabstock and molded. Slabstock foams are used in the carpet, furniture and bedding industries. Primary uses of slabstock foam are as carpet underlay and furniture padding. In the molded foam area, high resiliency (HR) molded foam is the foam type generally made. HR molded foams are used in the automotive industry for a breadth of applications ranging from molded seats to energy-absorbing padding and the like.
The basic patents relating to such polymer polyol compositions are Stamberger, U.S. Pat. No. Re. 28,715 (reissue of U.S. Pat. No. 3,383,351) and U.S. Pat. No. Re. 29,118 (reissue of U.S. Pat. No. 3,304,273). As described therein, a stable dispersion of polymer particles in a polyol can be produced by polymerizing one or more ethylenically unsaturated monomer dissolved or dispersed in a polyol in the presence of a free radical catalyst.
Initially, the primary polymer polyol compositions accepted commercially used acrylonitrile in its manufacture. Many of these compositions possessed undesirably high viscosities for certain applications. More recently, acrylonitrile-styrene monomer mixtures have been used commercially to make the polymer component of polymer polyols. The expanding demand for polymer polyols has highlighted several product needs and this has spawned additional advances in technology.
Polymer polyols derived from such high styrene monomer mixtures appear incapable of satisfying ever-increasing market needs, which include rigorous stability requirements and increased load-bearing characteristics in foams. Polymer polyols with increased load-bearing characteristics can be obtained by increasing their polymer or solid contents. Solid contents of 30 to 60 weight percent, or higher, are desired. Yet, the art has not been capable of increasing solid contents without reducing the stability of the polymer polyol and undesirably increasing its viscosity.
Employment of high styrene monomer mixtures and high solid content polymer polyols, by prior practices, generally resulted in undesirably high viscosity polymer polyols. The viscosity of a polymer polyol should be sufficiently low for ease of handling during its manufacture. In addition, the viscosity should facilitate transport, handling and, ultimately, adequate processability, in the employed foam processing equipment. Because of increased use of sophisticated mixing systems, such as impingement systems, excessive viscosity of the polymer polyol is becoming a significant problem. The need for lower viscosity polymer polyols is apparent to satisfy these increased demands in the art.
As indicated, polymer polyol stability is a concern to makers of polyurethanes. At one time, seediness or filterability, a measure of stability of polymer polyols, was not a major issue in commercial practices. However, advances in the state of the art of polyurethane production have resulted in revisions in polymer polyol stability criteria, especially in the molded foam area.
With commercial developments in sophisticated, high-speed and large-volume equipment and systems for handling, mixing and reacting polyurethane-forming ingredients have evolved the need for highly stable and low viscosity polymer polyols. Polymer polyols have certain minimum requirements for satisfactory processing in such sophisticated foam equipment. Typically, the prime requirement is that the polymer polyols possess sufficiently small particles so that filters, pumps and the like do not become plugged or fouled in relatively short periods of time.
Though there have been advances in reduction in viscosity and increase in solids of polymer polyols, there remains a need for improvement in viscosity reduction and increase in solids content. Greater reductions in viscosity are needed to meet market demands and greater effective increases in solids content are also needed by the market. More importantly, there is a need for technology in polymer polyols that maximizes viscosity reduction while also providing a viable mechanism to higher solids content.
U.S. Pat. No. 4,208,314 describes low viscosity polymer polyols made from acrylonitrile-styrene monomer mixtures. These polymer polyols are convertible to low density, water-blown polyurethane foams having reduced scorch, especially with relatively low acrylonitrile-to-styrene ratios. This reference also discloses a process for making polymer polyols with reduced particulates.
Enhanced stability of polymer polyols is believed to be provided by the presence of a minor amount of a graft or addition copolymer formed in situ from growing polymer chains and polyol molecules. Some prior approaches incorporate small amounts of unsaturation into the polyol in addition to that inherently present in the polyoxyalkylene polyols typically used in forming polymer polyols. It was believed that improved stability resulted due to an increased amount of an addition copolymer stabilizer expected to be formed. U.S. Pat. Nos. 3,652,639, 3,823,201, and 3,850,861, British Patent 1,126,025 and Japanese Patent Nos. 52-005887 and 48-101494 utilize this approach. The use of “stabilizer precursors,” also termed a “macromer” that contains a particular level of reactive unsaturation, is based on the expectation that during polymerization, in the preparation of the polymer polyol, adequate amounts of stabilizer will be formed by the addition polymerization of the precursor stabilizer with a growing polymer chain.
The general concept of using stabilizer precursors in polymerization is disclosed in, for example, U.S. Pat. Nos. 4,454,255 and 4,458,038. The macromer in these patents may be obtained by reacting a polyol with a compound having reactive ethylenic unsaturation such as, for example, maleic anhydride or fumaric acid. Another reference which describes this technique is U.S. Pat. No. 4,460,715. The reactive unsaturation in the '715 stabilizer is provided by an acrylate or methacrylate moiety.
U.S. Pat. No. 4,242,249 discloses improved polymer polyols prepared by utilizing certain preformed dispersants or preformed stabilizers. These polymer polyols provide stability satisfactory for commercial production, and use of at least one of the following: (1) higher amounts of styrene or other comonomer when acrylonitrile copolymer polymer polyols are being prepared, (2) higher polymer contents or (3) lower molecular weight polyols.
Other references which describe stabilizer precursors (or macromers) for polymer polyols include, for example, U.S. Pat. Nos. 4,550,194, 4,652,589, and 4,997,857. The stabilizer precursors of U.S. Pat. No. 4,997,857 are characterized by these four features: (1) they are prepared from a starting polyol having a functionality greater than 4;(2) they have at least 60% retained unsaturation; (3) they have viscosities greater than 2000 centipoise at 25° C.; and (4) the starting polyol is capped with ethylene oxide and/or the adduct formed between the starting polyol and the a reactive unsaturated compound is capped with ethylene oxide.
Other references which describe polymer polyols and/or processes of making polymer polyols include, for example, Simroth et al., U.S. Pat. No. Re. 32,733;Ramlow et al., U.S. Pat. No. 3,931,092; Ramlow et al., U.S. Pat. No. 4,014,846; Ramlow et al., U.S. Pat. No. 4,093,573; Shah, U.S. Pat. No. 4,148,840; Shook et al., U.S. Pat. No. 4,172,825; Kozawa et al., U.S. Pat. No. 4,342,840; Hoffman et al., U.S. Pat. No. 4,390,645; Hoffman, U.S. Pat. No. 4,394,491; Ramlow et al., U.S. Pat. No. 4,454,255; Ramlow et al., U.S. Pat. No. 4,458,038; and Hoffman, U.S. Pat. No. 4,745,153.
A pre-formed stabilizer (PFS) is particularly useful for preparing a polymer polyol having a lower viscosity at a high solids content. In the pre-formed stabilizer processes, a macromer is reacted with monomers to form a co-polymer of composed of macromer and monomers. These co-polymers comprising a macromer and monomers are commonly referred to as pre-formed stabilizers (PFS). Reaction conditions may be controlled such that a portion of the co-polymer precipitates from solution to form a solid. In many applications, a dispersion having a low solids content (e.g., 3 to 15% by weight) is obtained. Preferably, the reaction conditions are controlled such that the particle size is small, thereby enabling the particles to function as “seeds” in the polymer polyol reaction.
For example, U.S. Pat. No. 5,196,476 discloses a pre-formed stabilizer composition prepared by polymerizing a macromer and one or more ethylenically unsaturated monomers in the presence of a free-radical polymerization initiator and a liquid diluent in which the pre-formed stabilizer is essentially insoluble. EP 0,786,480 discloses a process for the preparation of a pre-formed stabilizer by polymerizing, in the presence of a free-radical initiator, from 5 to 40% by weight of one or more ethylenically unsaturated monomers in the presence of a liquid polyol comprising at least 30% by weight (based on the total weight of the polyol) of a coupled polyol which may contain induced unsaturation. These pre-formed stabilizers can be used to prepare polymer polyols which are stable and have a narrow particle size distribution. The coupled polyol is necessary to achieve a small particle size in the pre-formed stabilizer, which preferably ranges from 0.1 to 0.7 micron. U.S. Pat. Nos. 6,013,731 and 5,990,185 also disclose pre-formed stabilizer compositions comprising the reaction product of a polyol, a macromer, at least one ethylenically unsaturated monomer, and a free radical polymerization initiator.
Polymer control agents, also commonly called reaction moderators, are well known and are commonly used in the preparation of polymer polyols as is described in, for example, U.S. Pat. No. Re 33,291, (reissue of U.S. Pat. No. 4,454,255), U.S. Pat. Nos. 4,652,589, 5,196,476, 5,814,699, 5,990,185, and 6,455,603. Preformed stabilizers are used in the polymer polyols of U.S. Pat. Nos. 5,196,476 and 5,990,185, however, these do not use greater than 5% by weight of a polymer control agent. In fact, the trend has been towards decreasing the quantity of polymer control agent in making polymer polyols as in U.S. Pat. No. 6,455,603. Decreasing the amount of PCA decreases the amount of volatiles that need to be stripped at the end of the reaction. In addition, it was previously thought that too high of a level of PCA resulted in polyurethane foams with decreased physical properties. Thus, the quantity of PCA was kept below 5% by weight.
U.S. Pat. No. 4,652,589 describes preparative techniques that allow a polymer polyol to be prepared with the “indigenous viscosity” for the particular system, i.e. the minimum product viscosity for a given polymer polyol under the particular reaction conditions. This reduction in product viscosity is accompanied by an observable change in the somewhat rough surfaces of the polymer particles to a predominance (i.e. at least a majority) of particles appearing to have relatively smooth exterior surfaces. The general concept of these techniques is to increase the fluidity of the particles to obtain at least a predominance of smooth particles with a concurrent reduction in product viscosity. Increased fluidity can be obtained by, for example, the presence of a polymer control agent. This is illustrated in Example 9 of U.S. Pat. No. 4,652,589. Example 9 prepares a polymer polyol by feeding polyol, catalyst, styrene, acrylonitrile, a macromer, and 4.6% methanol (a PCA), based on the total weight of the reactor feeds, to a continuous-stirred tank reactor. This amount of methanol is sufficient to produce a polymer polyol in which the vast majority of the particles had relatively smooth surfaces, and the polymer polyols is therefore assumed to have its lowest viscosity or “indigenous” viscosity.
Surprisingly, and contrary to the disclosure of U.S. Pat. No. 4,652,589, we have found that increasing the level of polymer control agent above the level which is sufficient to produce a majority of smooth particles effects a further reduction in the viscosity of the polymer polyol. The exact mechanism by which the additional polymer control agent lowers the viscosity of a polymer polyol that, except for the added PCA, would already have smooth particle surfaces and thus exhibit its “indigenous” viscosity is not completely understood. Furthermore, it was surprisingly found that increasing the quantity of PCA enables the preparation of a higher solids content polymer polyol with a lower viscosity while maintaining good filterability. Thus, it appears that the higher levels of PCA are more effective at higher solids contents.